U.S. patent application number 10/864908 was filed with the patent office on 2004-12-23 for electrochemical urea sensors and methods of making the same.
Invention is credited to Cosofret, Vasile V., Shin, Jungwon.
Application Number | 20040256227 10/864908 |
Document ID | / |
Family ID | 34972143 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040256227 |
Kind Code |
A1 |
Shin, Jungwon ; et
al. |
December 23, 2004 |
Electrochemical urea sensors and methods of making the same
Abstract
An electrochemical sensor for detecting urea in a sample
includes a composite membrane that includes an enzymatic layer and
an outer diffusional layer. The outer diffusional layer is disposed
between the enzymatic layer and the analytical sample, preventing
direct contact between the enzymatic layer and the analytical
sample.
Inventors: |
Shin, Jungwon; (Waltham,
MA) ; Cosofret, Vasile V.; (Acton, MA) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
34972143 |
Appl. No.: |
10/864908 |
Filed: |
June 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10864908 |
Jun 9, 2004 |
|
|
|
10364840 |
Feb 11, 2003 |
|
|
|
Current U.S.
Class: |
204/403.03 ;
427/402 |
Current CPC
Class: |
G01N 33/54393 20130101;
C12Q 1/005 20130101; G01N 33/492 20130101; G01N 33/62 20130101;
G01N 33/5438 20130101 |
Class at
Publication: |
204/403.03 ;
427/402 |
International
Class: |
B05D 001/36; B05D
007/00; G01N 031/00 |
Claims
What is claimed is:
1. A sensor for detecting urea in a sample, comprising: an
electrode; an enzymatic layer; and an outer diffusional layer
disposed between the sample and the enzymatic layer, wherein the
outer diffusional layer is permeable to urea.
2. The sensor of claim 1, wherein the electrode comprises a metal
electrode.
3. The sensor of claim 2, wherein the electrode comprises a
silver/silver chloride electrode.
4. The sensor of claim 1, wherein the enzymatic layer catalyzes the
hydrolysis of urea to ammonium ions.
5. The sensor of claim 4, wherein the enzymatic layer comprises
urease.
6. The sensor of claim 1, wherein the enzymatic layer further
comprises one or more enzyme stabilizers.
7. The sensor of claim 6, wherein the enzyme stabilizer is selected
from the group consisting of polypeptides and inert proteins.
8. The sensor of claim 7, wherein the polypeptide comprises
glutathione.
9. The sensor of claim 7, wherein the inert protein comprises
bovine serum albumin.
10. The sensor of claim 1, wherein the enzymatic layer comprises
urease cross-linked to glutathione.
11. The sensor of claim 10, further comprising one or more inert
proteins cross-linked to at least one of the urease and the
glutathione.
12. The sensor of claim 11, wherein the inert protein comprises
bovine serum albumin.
13. The sensor of claim 1, wherein the outer diffusional layer
comprises a polymer.
14. The sensor of claim 13, wherein the polymer is selected from
the group consisting of polyurethane, a poly(tetrafluoroethylene)
ionomer, poly-(2-hydroxymethyl methacrylate), polyvinyl chloride,
carboxylated polyvinyl chloride, hydroxylated polyvinyl chloride,
polycarbonate, cellulose acetate, and mixtures thereof.
15. The sensor of claim 14, wherein the polymer comprises
polyurethane.
16. The sensor of claim 1, further comprising an ion-selective
membrane disposed between the electrode and the enzymatic
layer.
17. The sensor of claim 16, wherein the ion-selective membrane
comprises an ammonium ion-selective membrane.
18. The sensor of claim 17, wherein the ion-selective membrane
comprises: a polymer matrix; and an ammonium-selective ionophore
disposed therein.
19. The sensor of claim 18, wherein the polymer matrix is selected
from the group consisting of polyvinyl chloride, carboxylated
polyvinyl chloride, hydroxylated polyvinyl chloride, polyurethane,
poly(tetrafluoroethylene), poly(methyl methacrylate), silicone
rubber, and mixtures thereof.
20. The sensor of claim 19, wherein the polymer matrix comprises
polyvinyl chloride.
21. The sensor of claim 18, wherein the ionophore is selected from
the group consisting of nonactin, monactin, dinactin, trinactin,
tetranactin, narasin, hexaoxaheptacyclotritrtracontane, benzocrown
ethers, cyclic depsipeptides, and mixtures thereof.
22. The sensor of claim 21, wherein the ionophore comprises
nonactin.
23. The sensor of claim 1, wherein the sample is selected from the
group consisting of blood, serum, plasma, cerebro-spinal fluid,
saliva, and urine.
24. The sensor of claim 23, wherein the sample is blood.
25. A sensor card comprising: a substrate; and one or more sensors
of claim 1 disposed on or in the substrate.
26. A method of forming the sensor of claim 1, the method
comprising the steps of: providing an electrode; applying to at
least one surface of the electrode an enzymatic layer; and applying
an outer diffusional layer over the enzyme layer, wherein the
sensor detects or measures urea in a sample.
27. A method of forming the sensor of claim 15, the method
comprising the steps of: providing an electrode; applying to at
least one surface of the electrode an ion-selective membrane;
applying to the ion-selective membrane an enzymatic layer; and
applying an outer diffusional layer over the enzyme layer, wherein
the sensor detects or measures urea in a sample.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/364,840, filed Feb. 11, 2003, the enclosure
of which is incorporated by reference herein.
TECHNICAL FIELD
[0002] The present invention is related to the field of
electrochemical sensors, particularly to sensors that measure urea
in analytical samples.
BACKGROUND INFORMATION
[0003] Researchers and clinicians often need to measure the
concentration of various analytes in biological samples. These
analytes include dissolved gases (e.g. carbon dioxide), ions (e.g.
hydrogen, sodium, potassium, calcium, lithium, ammonium, and
magnesium), and biologically active molecules (e.g. urea). In many
cases, the biological sample is a body fluid (e.g., blood, serum,
plasma, cerebro-spinal fluid, saliva, and/or urine) taken from a
patient during an office visit or while undergoing surgery. Proper
diagnosis and treatment often depend upon the accuracy of these
measurements and the speed with which they are obtained.
[0004] An electrochemical sensor system is an analytical tool that
can be used to measure the concentration of an analyte in a
biological sample. The electrochemical sensor contains a physical
transducer, such as a metal electrode, separated from the
analytical sample by at least one semi-permeable membrane. Enzyme
sensors, which detect and/or measure biological metabolites (such
as carbohydrates, peptides, lipids, nucleotides, or urea, for
example) in an analytical sample, include an enzymatic layer that
covers the semi-permeable membrane. The enzymatic layer contains at
least one enzyme that reacts with the analyte of interest to
produce a chemical byproduct, which can migrate through the
semi-permeable membrane and be detected by the electrode. An
example of an enzyme sensor is a urea sensor, which can contain an
enzyme, such as urease, that hydrolyzes urea to form ammonium
ions.
[0005] Existing urea sensors generally have a short lifespan
compared to other types of sensors (e.g., ion sensors). This is due
to the fact that the enzymatic layer of existing urea sensors is in
direct contact with the analytical sample, which allows the enzyme
and other components of the membrane to leach into the sample
and/or contaminants in the sample to degrade or destroy the enzyme
in the enzyme layer.
SUMMARY OF THE INVENTION
[0006] The present invention provides a urea sensor in which the
enzymatic layer is not in direct contact with the sample, which
results in a sensor that allows for multiple and continuous
determinations of urea concentration in samples. In addition, the
urea sensor of the present invention is amenable to miniaturization
and can be incorporated into sensor cards containing other
miniaturized electrochemical sensors used for continuous, long-term
analysis of multiple analytes without compromising the overall
useful life of the sensor card.
[0007] In general, in one aspect, the present invention features a
sensor for detecting urea in a sample that includes an electrode,
an enzymatic layer, and an outer diffusional layer. The outer
diffusional layer is permeable to urea and is disposed between the
enzymatic layer and the sample. By preventing direct contact
between the enzymatic layer and the sample, the outer diffusional
layer inhibits degradation and/or loss of the enzyme, thus
extending the life of the urea sensor. The urea sensor can be used
to determine the urea concentration of blood, serum, plasma,
cerebro-spinal fluid, saliva, and/or urine samples, for
example.
[0008] Embodiments of this aspect of the invention may include the
following features. The electrode may be a metal electrode, such as
a silver/silver chloride electrode. The enzymatic layer may contain
an enzyme, such as urease, that catalyzes the hydrolysis of urea to
ammonium ions. The enzymatic layer may also contain one or more
enzyme stabilizers, including, for example, polypeptides (e.g.,
glutathione) and inert proteins (e.g., bovine serum albumin). In a
particular embodiment, the enzymatic layer includes urease
cross-linked to glutathione. In other embodiments, the enzymatic
layer includes one or more inert proteins, such as bovine serum
albumin, cross-linked to at least one of urease and glutathione.
The outer diffusional layer can include a polymer, such as
polyurethane, a poly(tetrafluoroethylene) ionomer,
poly-(2-hydroxymethyl methacrylate), polyvinyl chloride,
carboxylated polyvinyl chloride, hydroxylated polyvinyl chloride,
polycarbonate, cellulose acetate, and mixtures thereof.
[0009] In some embodiments, the urea sensor further includes an
ion-selective membrane disposed between the electrode and the
enzymatic layer. For example, the ion-selective membrane can be an
ammonium ion-selective membrane that includes a polymer matrix in
which an ammonium-selective ionophore is disposed. The polymer
matrix can include, for example, polyvinyl chloride, carboxylated
polyvinyl chloride, hydroxylated polyvinyl chloride, polyurethane,
poly(tetrafluoroethylene), poly(methyl methacrylate), silicone
rubber, and mixtures thereof. Examples of suitable ammonium
ion-selective ionophores include nonactin, monactin, dinactin,
trinactin, tetranactin, narasin, hexaoxaheptacyclotritrtracontane,
benzocrown ethers, cyclic depsipeptides, and mixtures thereof.
[0010] In another aspect, the present invention features a sensor
card that includes one or more sensors described above disposed in
or on a substrate.
[0011] In yet another aspect, the present invention provides
methods for forming a sensor described above. In one approach, the
method includes the steps of providing an electrode, applying to at
least one surface of the electrode an enzymatic layer, and applying
an outer diffusional layer over the enzymatic layer. In another
approach, the method includes the steps of providing an electrode,
applying to at least one surface of the electrode an ion-selective
membrane, applying to the ion-selective membrane an enzymatic
layer, and applying an outer diffusional layer over the enzymatic
layer.
[0012] The foregoing and other objects, aspects, features, and
advantages of the invention will become more apparent from the
following description and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] In the drawings, like reference characters generally refer
to the same parts throughout the different views. Also, the
drawings are not necessarily to scale, emphasis instead generally
being placed upon illustrating the principles of the invention.
[0014] FIG. 1 is a schematic diagram of the components of an
embodiment of an electrochemical sensor system according to the
invention, including a sensor cartridge with an electrode card and
sample inlet, a peristaltic pump, and a microprocessor.
[0015] FIG. 2 illustrates a frontal view of an embodiment of an
electrode card according to the invention.
[0016] FIG. 3 illustrates a cross sectional view of an embodiment
of an ion-selective electrode (ISE) according to the invention.
[0017] FIG. 4 illustrates a cross sectional view of an embodiment
of a carbon dioxide (CO.sub.2) electrode according to the
invention.
[0018] FIG. 5 illustrates a cross sectional view of an embodiment
of an enzyme electrode according to the invention.
[0019] FIG. 6 is a table containing examples of polymeric membrane
components and their respective weight percentages for four
different ISEs.
[0020] FIG. 7 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes a
sodium ISE with a polymeric membrane containing high molecular
weight polyvinyl chloride (HMW-PVC), with measurements taken before
and after assaying a sodium-containing sample contaminated with 10
mg/dL thiopental sodium (thiopental).
[0021] FIG. 8 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes
five sodium ISEs with polymeric membranes containing carboxylated
polyvinyl chloride (PVC-COOH) according to the invention, with
measurements taken before and after assaying a sodium-containing
sample contaminated with 10 mg/dL thiopental.
[0022] FIG. 9 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes a
potassium ISE with a polymeric membrane containing HMW-PVC, with
measurements taken before and after assaying a potassium-containing
sample contaminated with 10 mg/dL thiopental.
[0023] FIG. 10 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes
five potassium ISEs with polymeric membranes containing PVC-COOH
according to the invention, with measurements taken before and
after assaying a potassium-containing sample contaminated with 10
mg/dL thiopental.
[0024] FIG. 11 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes a
calcium ISE with a polymeric membrane containing HMW-PVC, with
measurements taken before and after assaying a calcium-containing
sample contaminated with 10 mg/dL thiopental.
[0025] FIG. 12 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes
five calcium ISEs with polymeric membranes containing PVC-COOH
according to the invention, with measurements taken before and
after assaying a calcium-containing sample contaminated with 10
mg/dL thiopental.
[0026] FIG. 13 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes a
pH electrode with a polymeric membrane containing HMW-PVC, with
measurements taken before and after assaying a pH buffered sample
contaminated with 10 mg/dL thiopental.
[0027] FIG. 14 is a graphical representation of one of the
chronopotentiometric responses of an electrode card that includes
five pH electrodes with polymeric membranes containing PVC-COOH
according to the invention, with measurements taken before and
after assaying a pH buffered sample contaminated with 10 mg/dL
thiopental.
[0028] FIG. 15 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes a
CO.sub.2 electrode with a polymeric membrane containing HMW-PVC,
with measurements taken before and after assaying a sample
contaminated with 10 mg/dL thiopental.
[0029] FIG. 16 is a graphical representation of the
chronopotentiometric responses of an electrode card that includes
four CO.sub.2 electrodes with polymeric membranes containing
PVC-COOH according to the invention, with measurements taken before
and after assaying a sample contaminated with 10 mg/dL
thiopental.
[0030] FIG. 17 is a graphical representation of the bulk membrane
resistance for an ISE with a polymeric membrane containing
HMW-PVC.
[0031] FIG. 18 is a graphical representation of the bulk membrane
resistance for an ISE with a polymeric membrane containing
PVC-COOH.
[0032] FIG. 19 is a graphical representation comparing sodium
concentration values in whole blood samples determined by a sodium
ISE with a polymeric membrane containing HMW-PVC against those
determined by an electrode card including five sodium ISEs with
polymeric membranes containing PVC-COOH according to the
invention.
[0033] FIG. 20 is a graphical representation comparing potassium
concentration values in whole blood samples determined by a
potassium ISE with a polymeric membrane containing HMW-PVC against
those determined by an electrode card including five potassium ISEs
with polymeric membranes containing PVC-COOH according to the
invention.
[0034] FIG. 21 is a graphical representation of the negative drift
in potential over time of a sodium ISE with a polymeric membrane
containing PVC-COOH according to the invention.
[0035] FIG. 22 is a graphical representation of the response of a
urea sensor according to the invention to two different calibration
solutions.
[0036] FIG. 23 is a graphical representation comparing urea
concentration values in plasma samples determined by a commercially
available clinical chemistry analyzer against those in
corresponding whole blood samples determined by an electrode card
including a urea sensor according to the invention.
DESCRIPTION
[0037] An electrochemical sensor according to the invention can be
incorporated into an electrochemical sensor system. Referring to
FIG. 1, in one embodiment according to the invention, the
electrochemical sensor system 1 has an inlet 2 where the biological
sample is introduced into the electrochemical sensor system 1. A
peristaltic pump 4 moves a sample, such as a body fluid sample,
through the inlet 2 and into an electrode card 6. The electrode
card 6 contains one or more electrodes 8 that detect and measure
analytes of interest in the sample. An electrical interface 10
connects the electrode card 6 to a microprocessor 12. Signals from
the electrode card 6 pass to the microprocessor 12 to allow for
storage and display of the signals. Signals from the microprocessor
12 pass to the electrode card 6 to allow for control over
measurement conditions, such as the polarization voltage of an
electrode. In one embodiment according to the invention, the sample
inlet 2 and the electrode card 6 are contained within a disposable
cartridge 13, which can be detached from the remaining elements of
the electrochemical sensor system 1 and replaced after use.
[0038] Referring to FIG. 2, in one embodiment according to the
invention, the electrode card 6 includes a rigid, substantially
rectangular card made of polyvinyl chloride (PVC). A channel 20 is
located within the electrode card 6, through which a biological
sample or a reference solution can flow. One or more electrodes 8
can be embedded within the channel 20. When a sample is passed
through the electrode card 6, it flows through the channel 20 and
over the electrodes 8, allowing for detection and/or measurement of
the analyte(s) of interest.
[0039] Referring to FIG. 2, the electrodes 8 that can be
incorporated into the electrode card 6 include ion-selective
electrodes (ISEs) 100, electrodes for analyzing dissolved gases
(gas electrodes), and electrodes which use an enzyme-based
detection system (enzyme electrodes). For example, the electrodes
may detect sodium 26, calcium 28, potassium 30, pH 32, lithium 34,
magnesium 36, ammonium 38, carbon dioxide 40, and urea 42.
[0040] Referring to FIG. 3, in one embodiment according to the
invention, an ISE 100 comprises a metal element 105, an inner
solution layer 110, and a polymeric membrane 115. The metal element
105 is embedded in the PVC of an electrode card 6, and the inner
solution layer 110 covers the exposed end of the metal element 105.
The inner solution layer 110 may contain, for example,
2-[N-morpholino]ethanesulfonic acid (MES) buffer. The polymeric
membrane 115 is an ion-selective membrane that separates the inner
solution layer 110 from an analytical sample (for example, a body
fluid sample) that passes through the channel 20 in the electrode
card 6. The composition of the polymeric membrane 115 determines
the selectivity of the ISE 100 for a particular ion. In a
particular embodiment according to the invention, PVC-COOH is a
component of the polymeric membrane 115.
[0041] Referring back to FIG. 2, to measure the concentration of an
ion in an analytical sample, an ISE 100 must work in tandem with a
reference electrode 44. If the ion that the ISE 100 is designed to
detect is present in the analytical sample, an electrical potential
is generated across the polymeric membrane 115 that depends on the
difference between the concentration of the analyte in the inner
solution layer 110, illustrated in FIG. 3, and its concentration in
the analytical sample. The difference in electrical potential
between the ISE 100 and the reference electrode 44 is directly
proportional to the change in the logarithm of the concentration of
the measured ion in the analytical sample.
[0042] Referring to FIG. 4, in one embodiment according to the
invention, a carbon dioxide (CO.sub.2) electrode 40, one type of
gas electrode, comprises a metal element 125, an inner solution
layer 130, and a polymeric membrane 135. The CO.sub.2 electrode 40
is functionally similar to an ISE 100, except that the inner
solution layer 130 of the CO.sub.2 electrode 40 is bicarbonate
buffer. Referring to FIG. 2, unlike an ISE 100, the CO.sub.2
electrode 40 must work in tandem with a pH electrode 32.
[0043] Referring again to FIG. 4, when CO.sub.2 permeates the
polymeric membrane 135 of the CO.sub.2 electrode 40, it dissolves
in the bicarbonate buffer of the inner solution layer 130 and
changes the buffer pH, which changes the electrical potential of
the CO.sub.2 electrode 40. The inner solution layer of the pH
electrode 32, however, is not affected by CO.sub.2 in the
analytical sample, so the pH electrode's potential remains
constant. The difference in electrical potential between the
CO.sub.2 electrode 40 and the pH electrode 32 is proportional to
the concentration of CO.sub.2 in the sample. In one embodiment
according to the invention, PVC-COOH can be a constituent of the
polymeric membrane 135 of a CO.sub.2 electrode 40.
[0044] FIG. 5 illustrates another embodiment according to the
invention, an enzyme electrode 150 for detecting the presence and
concentration of biological metabolites (such as a carbohydrate,
peptide, lipid, nucleotide, or urea, for example) in an analytical
sample. The enzyme electrode 150 comprises a metal element 155
embedded in an electrode card 6 and a composite membrane 160, which
is located between the metal element 155 and an analytical sample
flowing through a channel 20 in the electrode card 6. The composite
membrane 160 includes an outer diffusional membrane 165 adjacent to
the channel 20, an enzymatic layer 170, an ion-selective polymeric
membrane 175, and an inner solution layer 180 adjacent to the metal
element 155. The outer diffusional membrane 165 controls the
diffusion of the analyte into the enzyme layer 170 and protects the
other components of the enzyme electrode 150 from direct contact
with the analytical sample in the channel 20. The enzyme layer 170
may include at least one enzyme, or a mixture of several enzymes,
proteins, and stabilizers, that reacts with a particular analyte.
If the analyte diffuses through the outer diffusional membrane 165,
it can react with the enzyme(s) in the enzyme layer 170 to produce
a chemical byproduct, which can migrate through the ion-selective
polymeric membrane 175. In the case of a urea sensor, the chemical
byproduct can be ammonium ions, for example. An electrical
potential is generated across the composite membrane 160 that
depends on the concentration of the chemical byproduct, which is
proportional to the concentration of the analyte of interest in the
analytical sample. In one embodiment according to the invention,
PVC-COOH is a constituent of the ion-selective polymeric membrane
175.
[0045] In a particular embodiment, a urea sensor according to the
invention includes an enzyme that catalyzes the hydrolysis of urea
to ammonium ions, which are then detected by an ammonium
ion-selective electrode. An example of a suitable enzyme is urease.
Referring again to FIG. 5, the composite membrane 160 of the urea
sensor includes an outer diffusional membrane 165, an enzymatic
layer 170 containing urease, an ammonium ion-selective polymeric
membrane 175, and an inner solution layer 180 adjacent to a metal
element 155.
[0046] The outer diffusional membrane 165 is formulated to serve
several functions. First, the outer diffusional membrane 165
secures the enzymatic layer 170 to the ion-selective polymeric
membrane 175, preventing the enzymatic layer 170 from being washed
away by the flow of calibration solution or sample over the enzyme
layer that would otherwise occur without the protective outer
diffusional membrane 165. Second, the outer diffusional membrane
165 restricts the diffusion of contaminants from the sample into
the enzymatic layer 170, advantageously extending the useful life
of the sensor. In a particular embodiment, the outer diffusional
membrane 165 is formulated to restrict the diffusion of urea into
the enzymatic layer 170, thus allowing a smaller but proportional
fraction of the urea in the sample to react with the enzyme.
Restricting the amount of urea that reaches the enzymatic layer 170
not only extends the linear range of the urea sensor, but also
inhibits the pH of the enzymatic layer 170 from increasing, which
further extends the upper linear range of the sensor.
[0047] The outer diffusional membrane 165 generally includes one or
more polymers. Suitable polymers include, but are not limited to,
polyurethane, poly(tetrafluoroethylene) ionomers (e.g., the
perfluorosulfonate ionomer sold under the tradename NAFION by E.I.
Du Pont De Nemours & Co., Wilmington, Del.),
poly-(2-hydroxymethyl methacrylate), polyvinyl chloride, polyvinyl
chloride derivatives (e.g., carboxylated polyvinyl chloride and
hydroxylated polyvinyl chloride), polycarbonate, cellulose acetate,
and mixtures and copolymers thereof. A preferred polymer for use in
the outer diffusional membrane 165 is hydrophilic polyurethane
(available, for example, from Thermedics, Inc., Woburn, Mass.). In
one embodiment, the outer diffusional membrane 165 includes a
mixture or co-polymer of two or more polymers. In another
embodiment, the outer diffusional membrane 165 includes two or more
distinct layers of identical or different polymers and/or identical
or different co-polymers.
[0048] The enzymatic layer 170 includes an enzyme which catalyzes
the reaction of urea to ammonium ions, such as, for example,
urease. In one embodiment, the enzymatic layer 170 includes one or
more enzyme stabilizers, including, for example, inert proteins
(e.g., serum proteins, including bovine serum albumin),
polypeptides (e.g., glutathione), and solvents. In a particular
embodiment, glutathione is used together with one or more inert
proteins to stabilize the urease in the enzyme layer 170. In
addition, the urease may be cross-linked to glutathione and/or one
or more inert proteins using a cross-linking agent (e.g.,
glutaraldehyde) to prevent migration of the enzyme and/or the
stabilizers out of the enzymatic layer 170. Cross-linking also
secures the enzymatic layer 170 to the underlying ion-selective
polymeric layer 175. During fabrication of the enzymatic layer 170,
the enzyme stabilizers are generally added to the solution
containing the enzyme prior to the addition of the cross-linking
agent to ensure the stabilizers are cross-linked together with the
enzyme.
[0049] The ammonium ion-selective polymeric layer 175 includes an
ammonium-selective ionophore disposed in a polymeric matrix.
Suitable ionophores include, but are not limited to, nonactin,
monactin, dinactin, trinactin, tetranactin, narasin,
hexaoxaheptacyclotritrtracontane, benzocrown ethers (see, e.g.,
Anal. Chem., 2000, 72, 4683-4688) cyclic depsipeptides (see, e.g.,
Anal. Chem., 2003, 75, 152-156), and mixtures thereof. The
polymeric matrix can include, for example, polyvinyl chloride,
polyvinyl chloride derivatives (e.g., carboxylated polyvinyl
chloride and hydroxylated polyvinyl chloride), polyurethane,
poly(tetrafluoroethylene), poly(methyl methacrylate), silicone
rubber, and mixtures and co-polymers thereof. In addition, the
ammonium ion-selective polymeric layer 175 can contain other
additives, such as plasticizers and lipophilic salt additives, as
discussed herein.
[0050] Referring again to FIG. 3, the polymeric membrane of an ISE
regulates the selectivity of the ISE 100 toward an analyte of
interest. The polymeric membrane 115 includes at least four
elements: a polymer, a plasticizer, an ionophore, and a lipophilic
salt additive.
[0051] In one embodiment according to the invention, PVC-COOH is a
polymer component of the polymeric membrane 115. PVC-COOH is
polyvinyl chloride (PVC) polymer that has a percentage of its
chlorine atoms replaced by carboxyl groups (COOH). In an embodiment
according to the invention, the PVC-COOH can contain between 0.1
and 5% COOH by weight, and in a particular embodiment of the
invention, the PVC-COOH contains 1.8% COOH by weight. The PVC-COOH
prevents lipophilic anionic species, (such as analgesics and
anesthetics, for example) in the analytical sample from permeating
the polymeric membrane 115 and interfering with the ISE 100. Such
lipophilic anionic species can include thiopental sodium
(thiopental), phenyloin, ibuprofen, fenoprofen, salicylate,
valproate, and .epsilon.-amino-caproate, for example. PVC-COOH can
be mixed with another polymer (such as HMW-PVC or polyurethane, for
example) to form the polymer component of the polymeric membrane.
In a particular embodiment of the invention, PVC-COOH is not mixed
with another polymer and is the only polymer component of the
polymeric membrane.
[0052] The polymeric membrane 115 that includes PVC-COOH also
exhibits enhanced adhesive properties to solid platforms, which is
important for the long life and potential stability of an
electrochemical sensor. In addition, an electrochemical sensor
employing PVC-COOH in its polymeric membrane 115 shows better
potential stability and reproducibility of sensor measurements due
to a significant reduction in the membrane resistance conferred by
the relatively polar polymeric membrane 115, as illustrated by
Example 6 below. The precision and accuracy of a PVC-COOH sensor is
comparable to known ISEs, including sensors based on high molecular
weight polyvinyl chloride (HMW-PVC), as illustrated by Example 7
below.
[0053] The plasticizer component of the polymeric membrane 115
provides ion mobility within the membrane that is necessary to
obtain effective ion transfer. The plasticizer must be compatible
with the polymer component and must be a solvent for the ionophore.
The plasticizer must also be sufficiently insoluble in water so
that it does not migrate significantly into an aqueous sample in
contact with the surface of the polymeric membrane 115. It is also
desirable that the plasticizer be substantially non-volatile to
extend the shelf-life of the electrode. Useful plasticizers include
bis(2-ethylhexyl) sebacate (DOS) and o-nitrophenyl octyl ether
(NPOE).
[0054] The ionophore used in the polymeric membrane 115 is capable
of selectively associating with a specific ion. This feature of the
ionophore is responsible for the ion-selectivity of an ISE.
Examples of suitable ionophores for a sodium ISE include methyl
monensin ester, calixarene derivatives, and other sodium-sensitive
compounds. A monocyclic antibiotic (such as valinomycin, for
example) can be used as an ionophore for a potassium ISE. An
ionophore for a calcium ISE may be, for example,
(-)-(R,R)-N,N'-(Bis(11-ethoxycarbonyl)undecyl)-N,N'-4,5-tetr-
amethyl-3,6-dioxaoctanediamide; Diethyl
N,N'-[(4R,5R)-4,5-dimethyl-1,8-dio-
xo-3,6-dioxaoctamethylene]-bis(12-methylaminododecanoate) (ETH
1001). An example of a suitable ionophore for a pH electrode and/or
a carbon dioxide electrode is tridodecylamine (TDDA).
[0055] The lipophilic salt additive used in the polymeric membrane
115 serves to reduce membrane resistance and to reduce anion
interference. Useful lipophilic salt additives include, for
example, potassium tetrakis(4-chlorophenyl)borate (KTpClPB) and
potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTTFPB).
The lipophilic salt additive, however, is not always essential to
the function of the polymeric membrane 115 and can be omitted when
certain analytes are targeted.
[0056] The efficiency of the polymeric membrane 115 in rejecting
lipophilic anion drug contaminants is enhanced when the polymeric
membrane 115 composition is optimized in terms of
PVC-COOH/plasticizer ratio and by a proper selection of the type
and ratios of ionophore and lipophilic salt additive. For example,
the polymeric membrane 115 for a sodium ISE may contain 25-35%
PVC-COOH, 60-70% DOS, 2-8% calixarene derivative, and 1-3% KTTFPB
by weight. The polymeric membrane 115 for a potassium ISE may
contain, for example, 25-35% PVC-COOH, 60-70% DOS, 1-5%
valinomycin, and 0-1% KTpClPB by weight. The polymeric membrane 115
for a calcium ISE may contain, for example, 25-35% PVC-COOH, 60-70%
1:1 DOS/NPOE, 1-5% ETH 1001, and 0.2-2% KTpClPB by weight. The
polymeric membrane 115 for a pH or CO.sub.2 ISE may contain, for
example, 25-35% PVC-COOH, 60-70% DOS, 2-7% TDDA, and 1-4% KTpClPB
by weight. FIG. 6 illustrates examples of particular embodiments of
suitable polymeric membrane 115 components and their respective
weight ratios for a variety of electrochemical sensors.
[0057] Referring still to FIG. 3, a polymeric membrane 115
according to the present invention can be formed by dissolving the
appropriate amounts of polymer, plasticizer, ionophore, and
lipophilic salt additive in a solvent, typically tetrahydrofuran
(THF) or cyclohexanone, and applying this solution to the exposed
surface of a metal element 105 embedded in an electrode card 6. For
example, a potassium ISE can be fabricated by mixing PVC-COOH (1.8
wt % COOH), DOS, valinomycin and KTpClPB according to the ratios
listed in FIG. 6 to make a total mass of 630-650 mg. The mixture is
dissolved in 3-3.5 mL THF, and 0.75 .mu.L of the solution is
applied to the exposed end of the metal element 105 (for example, a
chloridized silver wire) embedded in the electrode card 6. Once the
solvent evaporates, the same volume of membrane solution is applied
two additional times with adequate drying time in between each
application. Once the solvent has evaporated from the last
application, the polymeric membrane 115 is formed and is bonded to
the electrode card 6.
[0058] Referring again to FIG. 5, in one embodiment, a urea sensor
according to the present invention is fabricated by first applying
the inner solution layer 180 (e.g., a MES buffer solution) to the
exposed surface of a metal element 155 (e.g., silver/silver
chloride or platinum) embedded in an electrode card 6. Next, a
solution of nonactin, polyvinyl chloride, and one or more
plasticizers is added over the inner solution layer 180 to form the
ion-selective polymer layer 175. The enzyme layer 170 is formed by
adding a solution urease, glutathione, bovine serum albumin, and
glutaraldehyde to the surface of the ion-selective polymer layer
175. Finally, a solution of hydrophilic polyurethane is added over
the enzyme layer 170 to form the outer diffusional layer 165.
[0059] An ISE according to the present invention measures changes
in the electric potential, measured in millivolts (mV), of an
analytical sample that are due to changes in the concentrations of
analytes within the sample. Similarly, a gas electrode according to
the present invention measures changes in the electric potential of
an analytical sample that are due to changes in the partial
pressure of the gas dissolved in the sample. As practitioners
skilled in the art are aware, electric potential values are related
to concentration or partial pressure values according to the Nernst
equation. In a particular embodiment according to the invention,
software may be included in the electrochemical sensor system to
convert electrical potential values measured by the electrode to
concentration or partial pressure values of the measured analyte by
using the Nernst equation.
[0060] In another aspect, the invention is a method for detecting
the presence and/or measuring the concentration of an analyte in a
body fluid (such as blood, for example) in the presence of a
lipophilic anionic species (such as thiopental, for example)
without interference by the lipophilic anionic species in detecting
and/or measuring the analyte. The method of the invention provides
an electrochemical sensor for detecting and/or measuring an analyte
of interest in a body fluid that includes PVC-COOH as a polymer
component of the sensor's polymeric membrane. A body fluid sample
containing the analyte of interest and a lipophilic anionic
contaminant is placed in contact with the electrochemical sensor
that includes PVC-COOH as a polymer component of the sensor's
polymeric membrane. The analyte of interest in the body fluid
sample is then measured and/or detected by the electrochemical
sensor with reduced interference by the lipophilic anionic
contaminant.
[0061] The following examples are intended to illustrate, but not
limit, the invention.
EXAMPLE 1
[0062] FIG. 7 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including a sodium ISE with a polymeric membrane containing
HMW-PVC. An analytical sample containing a known concentration of
sodium was introduced to the electrode card, and the ISE measured
the concentration of sodium to be 142 mM. At time t, the analytical
sample was changed to a solution containing the same concentration
of sodium plus 10 mg/dL thiopental, and the ISE returned a sodium
concentration value of 137 mM.
[0063] FIG. 8 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including five sodium ISEs with polymeric membranes containing
PVC-COOH according to the invention. An analytical sample
containing a known concentration of sodium was introduced to the
electrode card, and all five ISEs measured the concentration of
sodium to be 139 mM. At time t, the analytical sample was changed
to a solution containing the same concentration of sodium plus 10
mg/dL thiopental, and all five electrodes returned sodium
concentration values of 139 mM. The PVC-COOH based sodium ISEs did
not exhibit the drift in potential displayed by the HMW-PVC based
sodium ISE after analyzing a sample contaminated with thiopental,
which illustrates the efficacy of PVC-COOH in preventing
interference with sodium measurements caused by lipophilic anionic
contaminants.
EXAMPLE 2
[0064] FIG. 9 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including a potassium ISE with a polymeric membrane containing
HMW-PVC. An analytical sample containing a known concentration of
potassium was introduced to the electrode card, and the ISE
measured the concentration of potassium to be 3.2 mM. At time t,
the analytical sample was changed to a solution containing the same
concentration of potassium plus 10 mg/dL thiopental, and the ISE
returned a potassium concentration value of 2.8 mM.
[0065] FIG. 10 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including five potassium ISEs with polymeric membranes containing
PVC-COOH according to the invention. An analytical sample
containing a known concentration of potassium was introduced to the
electrode card, and all five ISEs measured the concentration of
potassium to be 3.3 mM. At time t, the analytical sample was
changed to a solution containing the same concentration of
potassium plus 10 mg/dL thiopental, and all five electrodes
returned potassium concentration values of 3.3 mM. The PVC-COOH
based potassium ISEs exhibited negligible potential drifts after
analyzing a sample contaminated with thiopental as compared to the
drift displayed by the HMW-PVC based potassium ISE, which
illustrates the efficacy of PVC-COOH in preventing interference
with potassium measurements caused by lipophilic anionic
contaminants.
EXAMPLE 3
[0066] FIG. 11 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including a calcium ISE with polymeric membrane containing HMW-PVC.
An analytical sample containing a known concentration of calcium
was introduced to the electrode card, and the ISE measured the
concentration of calcium to be 0.93 mM. At time t, the analytical
sample was changed to a solution containing the same concentration
of calcium plus 10 mg/dL thiopental, and the ISE returned a calcium
concentration value of 0.81 mM.
[0067] FIG. 12 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including five calcium ISEs with polymeric membranes containing
PVC-COOH according to the invention. An analytical sample
containing a known concentration of calcium was introduced to the
electrode card, and all five ISEs measured the concentration of
calcium to be 0.87 mM. At time t, the analytical sample was changed
to a solution containing the same concentration of calcium plus 10
mg/dL thiopental, and the five electrodes returned calcium
concentration values of 0.85, 0.85, 0.86, 0.86, and 0.85 mM. The
PVC-COOH based calcium ISEs did not exhibit as great a drift in
potential as displayed by the HMW-PVC based calcium ISE after
analyzing a sample contaminated with thiopental, which illustrates
the efficacy of PVC-COOH in preventing interference with calcium
measurements caused by lipophilic anionic contaminants.
EXAMPLE 4
[0068] FIG. 13 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including a pH electrode with a polymeric membrane containing
HMW-PVC. An analytical sample containing a buffer solution of known
pH was introduced to the electrode card, and the electrode measured
the pH to be 7.63. At time t, the analytical sample was changed to
a solution containing the same buffer solution plus 10 mg/dL
thiopental, and the electrode returned a pH value of 7.68.
[0069] FIG. 14 is a graphical representation of one of the
chronopotentiometric responses recorded from an electrode card
including five pH electrodes with polymeric membranes containing
PVC-COOH according to the invention. An analytical sample
containing a buffer solution of known pH was introduced to the
electrode card, and all five electrodes measured the pH to be 7.66.
At time t, the analytical sample was changed to a solution
containing the same buffer solution plus 10 mg/dL thiopental, and
the five electrodes returned pH values of 7.68. The PVC-COOH based
pH electrodes did not exhibit as great a drift in potential as
displayed by the HMW-PVC based pH electrode after analyzing a
sample contaminated with thiopental, which illustrates the efficacy
of PVC-COOH in preventing interference with pH measurements caused
by lipophilic anionic contaminants.
EXAMPLE 5
[0070] FIG. 15 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including a CO.sub.2 electrode with polymeric membrane containing
HMW-PVC. An analytical sample containing a buffer solution was
introduced to the electrode card. Two measurements of the partial
pressure of CO.sub.2 in the solution were taken, and the results
were averaged to yield a value of 66 mm Hg. At time t, the
analytical sample was changed to a solution containing the same
buffer solution plus 10 mg/dL thiopental, and the electrode
returned CO.sub.2 partial pressure value of 115 mm Hg.
[0071] FIG. 16 is a graphical representation of the
chronopotentiometric responses recorded from an electrode card
including four CO.sub.2 electrodes with polymeric membranes
containing PVC-COOH according to the invention. An analytical
sample containing a buffer solution was introduced to the electrode
card. Two measurements of the partial pressure of CO.sub.2 in the
solution were taken with each of the four electrodes, and the
results for each electrode were averaged to yield values of 67.9,
68.0, 68.0, and 68.2 mm Hg. At time t, the analytical sample was
changed to a solution containing the same buffer solution plus 10
mg/dL thiopental, and the four electrodes returned CO.sub.2 partial
pressure values of 70.4, 68.8, 68.8, and 73.4 mm Hg. The PVC-COOH
based CO.sub.2 electrodes display negligible potential drifts after
analyzing a sample contaminated with thiopental as compared to the
drift displayed by the HMW-PVC based CO.sub.2 electrode, which
illustrates the efficacy of PVC-COOH in preventing interference
with CO.sub.2 measurements caused by lipophilic anionic
contaminants.
EXAMPLE 6
[0072] FIG. 17 is a graphical representation of the bulk membrane
resistance of an ISE polymeric membrane that includes HMW-PVC as a
polymer component, as is known in the art. According to FIG. 17,
the HMW-PVC membrane has a bulk resistance of about
2.4.times.10.sup.7 ohms. By comparison, FIG. 18 shows that the bulk
membrane resistance of an ISE polymeric membrane that includes
PVC-COOH as a polymer component according to the invention is about
1.25.times.10.sup.6 ohms, which is over nineteen times lower than
that of an ISE containing HMW-PVC. By lowering the bulk membrane
resistance, an ISE fabricated using PVC-COOH in its polymeric
membrane exhibits enhanced potential stability and reproducibility
of measurements.
EXAMPLE 7
[0073] FIG. 19 is a graphical representation comparing sodium
concentration values in whole blood samples determined by a known
sodium ISE with a polymeric membrane containing HMW-PVC against
those determined by an electrode card including five sodium ISEs
with polymeric membranes containing PVC-COOH according to the
invention. Sodium concentration values for ninety-nine different
whole blood samples representing a wide range of sodium levels were
first determined with a HMW-PVC based sodium ISE, then with a
PVC-COOH based sodium ISE. As illustrated by FIG. 19, the values
obtained from the PVC-COOH based sodium ISE correlate well with
those obtained using the HMW-PVC based sodium ISE (r=0.9983), which
indicates the PVC-COOH based sodium ISE measures sodium with equal
precision and accuracy as a known sodium electrode.
[0074] FIG. 20 illustrates a similar experiment using a PVC-COOH
based potassium ISE. Potassium concentration values for 127
different whole blood samples representing a wide range of
potassium levels were first determined using a HMW-PVC based
potassium ISE, then a PVC-COOH based potassium ISE. As illustrated
by FIG. 20, the values obtained from the PVC-COOH based potassium
ISE correlate well with those obtained using the HMW-PVC based
electrode (r=0.9995), which indicates that the PVC-COOH based
potassium ISE measures potassium with equal precision and accuracy
as a known potassium electrode.
EXAMPLE 8
[0075] FIG. 21 is a graphical representation of the stability over
time of a PVC-COOH based sodium ISE according to the invention. On
day 1, an analytical sample containing a known concentration of
sodium was introduced to an electrode card including five sodium
ISEs with polymeric membranes containing PVC-COOH according to the
invention, and the concentration of sodium was measured. Then the
analytical sample was changed to a solution containing the same
concentration of sodium plus 10 mg/dL thiopental, and the
concentration of sodium was measured again. The difference between
the two measurements (AEMF), which represents the drift in the
electrode's potential due to thiopental interference, was
calculated, and the analytical sample was removed from the sensor
card. This procedure was repeated on days 2, 4, 6, 9, 12, 14, and
20, and the AEMF values were calculated. As FIG. 21 illustrates,
the effect that a lipophilic anionic species such as thiopental has
on a PVC-COOH based sodium ISE according to the invention remains
very low throughout the life of the electrode.
EXAMPLE 9
[0076] A urea sensor was fabricated according to the invention as
follows. A MES buffer solution was prepared containing 50.9 mmol/L
MES potassium salt, 46.6 mmol/L MES free acid, and 2 mmol/L
potassium chloride, and 0.05 .mu.L of this solution was applied to
the end of a silver/silver chloride wire embedded in a PVC
electrode card. Next, an ion-selective membrane solution was
prepared by combining 150 mg PVC, 100 .mu.L dioctyl sebacate, 100
.mu.L 2-nitrophenyl octyl ether, 110 .mu.L of 1% potassium
tetrakis(4-chlorophenyl)borate in THF, and 300 .mu.L of 5% nonactin
in THF in 8 mL THF. Two 0.75 .mu.L aliquots of this solution were
applied sequentially over the MES buffer layer to form the
ion-selective membrane. Next, an enzyme solution was prepared
containing 50 mg/mL urease (Biozyme, San Diego, Calif.), 20 mg/mL
glutathione, 10 mg/mL bovine serum albumin, and 0.12%
glutaraldehyde in 0.1 M phosphate buffer at a pH of 7.2, and 0.075
.mu.L of this solution was applied to the ion-selective membrane.
Finally, an outer membrane solution was prepared containing 0.12
g/ml polyurethane (Thermedics, Inc., Woburn, Mass.) in THF, and 0.5
.mu.L of this solution was applied over the enzyme layer to form
the outer membrane. The urea sensor was assembled into a disposable
cartridge and tested on a GEM Premier 3000 analyzer
(Instrumentation Laboratory Company, Lexington, Mass.).
[0077] During use, urea sensors according to the invention are
automatically calibrated using calibration solutions enclosed in
the disposable cartridge according to a predetermined schedule
throughout the useful life of the sensor. Calibration is performed
to determine the sensitivity of the urea sensor to urea (i.e., the
slope of the urea response curve) and the sensor's selectivity
coefficient for urea over potassium. To determine the sensitivity
of urea sensors according to the invention, the urea sensor
described above was calibrated with two different calibration
solutions. The first calibration solution (Cal B), which contained
1.0 mmol/L urea and 2.0 mmol/L potassium, was introduced into the
sample channel of the cartridge, and the signal of the urea sensor
was recorded. Next, Cal B was replaced with the second calibration
solution (Cal D), which contained 7.5 mmol/L urea and 15 mmol/L
potassium, and the sensor response was recorded. Finally, the
calibration solution was switched back to Cal B, and the sensor
response was recorded again. The sensor is always soaked in the Cal
B solution, except during calibration and sample analysis. As shown
in FIG. 22, the urea sensor responded rapidly and in a stable
manner to changes in the urea and potassium concentrations, which
indicates that urea sensors according to the invention are capable
of rapid analysis of analytical samples.
[0078] Next, the analytical performance of the urea sensor was
evaluated by analyzing the urea concentration of sixty-two whole
blood samples tested over a period of three weeks. The blood
samples were tested on the fourth, ninth, fourteenth, and
twenty-second days of the study. For each test, heparinized whole
blood samples were collected from healthy individuals and spiked
with a concentrated urea solution to yield samples with urea
concentrations ranging from 5 to 70 mmol/L. The urea concentration
value of each sample then was determined using a urea sensor
according to the invention as described above. For comparison, each
aliquot was centrifuged after analysis to isolate the plasma, and
the plasma urea concentration was determined using a conventional
clinical chemistry analyzer (MONARCH, Instrumentation Laboratory
Company, Lexington, Mass.). The Monarch analyzer uses a
spectrophotometric method in combination with a bienzymatic
reaction of urease, glutamate dehydrogenase, and NADH to detect
urea concentration in a sample. Because the Monarch analyzer can
only measure up to 35 mmol/L urea, the plasma samples were diluted
with saline prior to analysis. The results of the experiments are
summarized in FIG. 23.
[0079] As FIG. 23 indicates, the urea concentration values obtained
from the urea sensor of the present invention correlate well with
those obtained using the Monarch analyzer (r=0.995), which
indicates the urea sensor of the present invention measures urea
with equal precision and accuracy as a conventional chemistry
analyzer. Urea sensors according to the present invention have the
added benefit of being less expensive and easier to use than the
Monarch analyzer. In addition, the experiments indicate that the
urea sensor has a useful life of at least three weeks.
EXAMPLE 10
[0080] Ammonium ion-selective membranes of urea sensors according
to the invention can be at least partially permeable to potassium
ions, which can lead to erroneous urea concentration values. One
method for correcting the urea concentration value for potassium
interference involves independently measuring the potassium
concentration in the sample (using, for example, a potassium ISE as
described herein), determining a correction factor based on the
potassium concentration and the selectivity coefficient of the urea
sensor for urea over potassium, and then subtracting the correction
factor from the measured urea concentration value to obtain the
actual urea concentration value of the sample.
[0081] To determine if potassium interference can be corrected for
using urea sensors according to the invention, two urea sensors
were fabricated according to the method described in Example 9 and
incorporated into a disposable cartridge that also contained a
potassium ISE. The selectivity coefficient of each urea sensor was
determined by calibrating with two calibration solutions: Cal A
(1.0 mmol/L urea, 7.5 mmol/L potassium) and Cal B (1.0 mmol/L urea,
2.0 mmol/L potassium). Five whole blood samples were spiked with
different amounts of potassium, yielding samples with potassium
concentrations that ranged from 8 to 24 mmol/L. An un-spiked sample
had a potassium concentration of 4.2 mmol/L and a urea
concentration of 5.0 mmol/L. Each sample was introduced to the
cartridge, and the potassium and urea concentration values were
recorded. The urea concentration values were corrected based on the
selectivity coefficient and the potassium concentrations. The
results of the experiments are summarized in Table 1, below.
1TABLE 1 Corrected Urea Corrected Urea Potassium Concentration
Concentration, Sensor 1 Concentration, Sen- (mmol/L) (mmol/L) sor 2
(mmol/L) 4.2 5.0 5.0 8.5 5.0 5.0 12.1 5.0 5.0 16.0 5.0 5.0 20.0 5.0
5.0 24.3 5.0 5.0
[0082] As Table 1 indicates, potassium interference can effectively
be eliminated for urea sensors according to the present
invention.
[0083] Variations, modifications, and other implementations of what
is described herein will occur to those of ordinary skill in the
art without departing from the spirit and the scope of the
invention as claimed. Accordingly, the invention is to be defined
not by the preceding illustrative description but instead by the
spirit and scope of the following claims.
* * * * *